8 The Role of Humidity, Temperature, and Atmospheric Composition in Maintaining Vegetable Quality During Storage L. VAN DEN BERG Division of Biological Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6
Rising energy costs favour local long term storage of vegetables over long distance transport from distant growing areas. The energy required to truck a ton of carrots from Florida or Texas, U.S.A., to Toronto, Canada, for example, is more than sufficient to store the carrots under refrigeration for one year. Local growers also benefit when storage of vegetables is profitable, a point not lost on politicians or local growers themselves. Hence, growers are encouraged to increase their storage capacity for vegetables. The increased interest in storage of vegetables has led to a demand for up-to-date and valid information on optimum storage conditions, especially temperatures, relative humidity and atmospheric composition (C.A. storage). While there is widespread agreement on the optimum storage temperature for many vegetables, the same cannot be said for relative humidity. Recommended levels of relative humidity for the storage of fresh vegetables have traditionally been a trade-off between desiccation of the products by a low humidity on the one hand and increased decay by a high humidity on the other (1). The recommended optimum level of relative humidity has gradually edged closer to 100%, because i t was found that decay was not quite as serious a problem at high humidity as i t once was thought to be. Nevertheless, there is still a great reluctance on the part of horticulturists and storage operators alike to increase the recommended humidity level to the saturation point (100%). It is feared that such a high humidity would lead to a drastic increase in decay, either because of the humidity level itself or because condensation of water on the product becomes unavoidable in practice at this level. The development of the jacketed storage (2-8) led to a practical and economical way of maintaining a saturated atmosphere in the load space and this put new urgency on the question of the optimum level of relative humidity for fresh produce. Also, use of other systems of direct humidifcation (9,10) and of plastic packaging (11) raised questions of optimum humidity levels. 0097-6156/81/0170-0095$05.00/ 0 © 1981 American Chemical Society
96
QUALITY
OF
SELECTED
FRUITS
AND
VEGETABLES
E x t e n s i v e s t o r a g e t e s t s w i t h , v e g e t a b l e s have t h e r e f o r e been made a t t h e N a t i o n a l R e s e a r c h C o u n c i l L a b o r a t o r i e s o f Canada ( 6 , 1 2 - 2 1 ) and r e s u l t s have l e d t o a b r o a d e r and b e t t e r u n d e r s t a n d i n g o f t h e role of humidity in maintaining vegetable q u a l i t y . T h i s p a p e r p r e s e n t s u n p u b l i s h e d and p u b l i s h e d i n f o r m a t i o n f r o m s t u d i e s i n N R C ' s l a b o r a t o r i e s on t h e e f f e c t o f r e l a t i v e h u m i d i t y on d e c a y and o t h e r q u a l i t y f a c t o r s d u r i n g s t o r a g e . V e g e t a b l e s i n c l u d e d were b e e t s , B r u s s e l s s p r o u t s , c a b b a g e , c a r r o t s , c a u l i f l o w e r , c e l e r y , Chinese cabbage, l e e k s , parsnips and r u t a b a g a s . T e s t s a l w a y s i n c l u d e d a c o m p a r i s o n between t h e " a t " o r n e a r s a t u r a t e d l e v e l o f r e l a t i v e h u m i d i t y ( 9 8 - 1 0 0 % ) and one o r more l o w e r l e v e l s ( 8 5 - 9 5 % ) . The l e n g t h o f s t o r a g e i n v o l v e d v a r i e d f r o m a few weeks f o r c a u l i f l o w e r up t o one y e a r f o r carrots. D u r i n g t h i s t i m e f r e q u e n t o b s e r v a t i o n s were made on w e i g h t l o s s , d e c a y , a p p e a r a n c e , c o l o r , f i r m n e s s and c r i s p n e s s . Many o f t h e t e s t s a l s o i n v o l v e d s e v e r a l s t o r a g e t e m p e r a t u r e s ( i n t h e r a n g e 0 t o 7°C) and m o d i f i e d a t m o s p h e r e s ( 0 . 5 - 1 0 % o x y g e n , 1-11% c a r b o n d i o x i d e ) . Some b a s i c s t u d i e s on t h e e f f e c t o f h u m i d i t y on s u r v i v a l and g r o w t h o f d e c a y - c a u s i n g molds and on v e g e t a b l e r e s i s t a n c e t o p a t h o g e n s were a l s o made. The p a p e r c o n c l u d e s w i t h a summary on t h e e f f e c t s o f r e l a t i v e h u m i d i t y , t e m p e r a t u r e and a t m o s p h e r i c c o m p o s i t i o n on s t o r a g e l i f e o f several vegetables. M a t e r i a l s and
Methods
Sample M a t e r i a l . V e g e t a b l e s were o b t a i n e d f r o m m a j o r g r o w i n g a r e a s i n t h e p r o v i n c e s o f P r i n c e Edward I s l a n d , New B r u n s w i c k , Quebec and O n t a r i o i n C a n a d a . C a r r o t s , c e l e r y , Chinese cabbage, l e e k s and p a r s n i p s were o b t a i n e d f r o m muck s o i l s ; b e e t s , B r u s s e l s s p r o u t s , c a b b a g e , c a u l i f l o w e r and r u t a b a g a s f r o m loamy c l a y s o i l s . C u l t i v a r s s e l e c t e d were t h o s e g e n e r a l l y s t o r e d i n t h e g r o w i n g areas. These v e g e t a b l e s were m a t u r e a t h a r v e s t t i m e , were h a n d l e d c a r e f u l l y a f t e r h a r v e s t , and were p l a c e d u n d e r t e s t c o n d i t i o n s u s u a l l y w i t h i n 24 h o u r s a f t e r h a r v e s t . They were n o t w a s h e d , c l e a n e d o r g r a d e d , e x c e p t f o r d i s c a r d i n g o b v i o u s l y damaged s p e c i m e n s and l o o s e s o i l . Storage Tests. T e s t s w i t h a b o u t 35L s a m p l e s were made i n g a s - and v a p o r - p r o o f p l y w o o d c a b i n e t s ( h o l d i n g two 35L s a m p l e boxes e a c h ) i n w h i c h t e m p e r a t u r e , r e l a t i v e h u m i d i t y and a t m o s p h e r i c c o m p o s i t i o n c o u l d be c o n t r o l l e d a c c u r a t e l y . The s t e r i l i z e d sample boxes were e q u i l i b r a t e d a t t h e s t o r a g e r e l a t i v e h u m i d i t y p r i o r t o t h e t e s t s t o a v o i d m o i s t u r e a b s o r p t i o n by t h e wood d u r i n g t e s t s (22). M o d i f i e d a t m o s p h e r e s were o b t a i n e d by f l u s h i n g c a b i n e t s c o n t i n u o u s l y w i t h t h e d e s i r e d gas m i x t u r e . L a r g e r s c a l e t e s t s , t o d e t e r m i n e t h e e f f e c t o f sample s i z e on d e c a y a t h i g h r e l a t i v e h u m i d i t y , were made i n s m a l l c o m p l e t e l y j a c k e t e d rooms, used w i t h o u t i n t e r n a l f o r c e d a i r c i r c u l a t i o n . D u r i n g t e s t s , t h e r e l a t i v e h u m i d i t y i n t h e open s p a c e was 97-99%
8.
V A N D E N BERG
Maintaining Vegetable Quality During Storage
97
and a i r t e m p e r a t u r e was c o n t r o l l e d w i t h i n ± 0 . 3 ° C . P r o d u c e was g e n e r a l l y s t o r e d i n p a l l e t boxes o f a common c o m m e r c i a l d e s i g n ( 1 . 2 χ 1.2 χ 0 . 9 m, 2 - 8 boxes p e r t e s t ) . O b s e r v a t i o n s were made on d e c a y , w e i g h t l o s s and q u a l i t y d u r i n g t h e s t o r a g e p e r i o d . Basic Aspects. The f u n d a m e n t a l s t u d i e s f o c u s s e d on t h e e f f e c t o f r e l a t i v e h u m i d i t y on h o s t - p a r a s i t e r e l a t i o n s h i p s d u r i n g d e c a y f o r m a t i o n . A s p e c t s s t u d i e d were e p i d e r m i s g r o w t h d u r i n g s t o r a g e , changes i n f u n g i s t a t i c p r o p e r t i e s o f c a r r o t s d u r i n g s t o r a g e , s u r v i v a l o f p a t h o g e n i c m i c r o o r g a n i s m s d u r i n g s t o r a g e (23) and p r o d u c t i o n a n d s t a b i l i t y o f p e c t o l y t i c enzymes by t h e s e m i c r o organisms ( 2 4 ) . Results
and D i s c u s s i o n
Beets. Beets l o s t moisture r e a d i l y a t l e s s than s a t u r a t e d h u m i d i t y l e v e l s ( T a b l e I ) , r e s u l t i n g i n s o f t spongy b e e t s . Beets s t o r e d a t 9 8 - 1 0 0 % RH s t a y e d s o l i d a n d f r e s h . The optimum t e m p e r a t u r e f o r r e d u c i n g d e c a y was 4 - 5 ° C , r a t h e r t h a n 0-1 o r 2 - 3 ° C . However, s p r o u t i n g i n c r e a s e d w i t h t e m p e r a t u r e b u t a s p r o u t i n h i b i t o r c o u l d be u s e d . M o d i f i e d atmospheres a t 0 - l ° C (about 1%0 » 5 - 6 % C 0 ) c a u s e d r a p i d d e v e l o p m e n t o f d e c a y , e s p e c i a l l y m o l d . Damage d u r i n g h a r v e s t , and stems a n d l e a v e s n o t removed d u r i n g harvest i n c r e a s e d decay. 2
2
Brussels sprouts. Quality losses i n Brussels sprouts c a u s e d by a l e s s t h a n s a t u r a t e d h u m i d i t y l e v e l became a p p a r e n t a f t e r a b o u t 6 weeks o f s t o r a g e . B r u s s e l s s p r o u t s l o s t some o f t h e i r g r e e n c o l o r , w i l t e d and s h r i v e l l e d , p a r t i c u l a r l y when s t o r e d on t h e s t a l k . A f t e r a b o u t t h r e e months o f s t o r a g e , d e c a y and t r i m m i n g l o s s e s were s u b s t a n t i a l l y h i g h e r a t t h e l o w e r humidity (Table I I ) . Cabbage. S t o r a g e o f cabbage a t a r e l a t i v e h u m i d i t y n e a r o r a t s a t u r a t i o n (98-100%) reduced decay ( t r i m m i n g l o s s ) , w e i g h t l o s s and c o l o r l o s s s u b s t a n t i a l l y i n c o m p a r i s o n w i t h s t o r a g e a t 9 0 - 9 5 % r e l a t i v e h u m i d i t y a t a l l t e m p e r a t u r e s s t u d i e d ( T a b l e I I I ) . As a r e s u l t o f t h e reduced weight l o s s a t t h e h i g h e r humidity (mostly l e s s t h a n 0 . 5 % p e r 30 d a y s a s compared t o a b o u t 2% p e r 30 days a t 9 0 - 9 5 % ) , cabbage r e m a i n e d f i r m and c r i s p d u r i n g i t s s t o r a g e l i f e and r e t a i n e d i t s g r e e n c o l o r l o n g e r . R o o t i n g and i n t e r n a l g r o w t h were n o t a f f e c t e d by r e l a t i v e h u m i d i t y b u t depended m a r k e d l y on temperature: s t o r a g e l i f e a t 3 . 5 - 4 . 5 ° C and 7 - 8 ° C was l i m i t e d t o 4 - 5 months a n d 2 - 3 months r e s p e c t i v e l y by i n t e r n a l g r o w t h and r o o t i n g ; a t 0 - 1 ° C t h e s e were n o t s i g n i f i c a n t a f t e r 7 m o n t h s . Low c o n c e n t r a t i o n s o f oxygen a n d h i g h c o n c e n t r a t i o n s o f c a r b o n d i o x i d e f u r t h e r r e t a r d e d t h e l o s s o f green c o l o r o f cabbage, and atmos p h e r e s c o n t a i n i n g 1-3% oxygen and 5% c a r b o n d i o x i d e c a u s e d an a d d i t i o n a l small r e d u c t i o n i n decay. These r e s u l t s w i t h m o d i f i e d atmospheres agree w i t h f i n d i n g s i n t h e l i t e r a t u r e ( 2 5 , 2 6 ) . Oxygen l e v e l s b e l o w 1%, p a r t i c u l a r l y a t t e m p e r a t u r e s a t o r
98
QUALITY
Table
I.
Effect decay
of
relative
and w e i g h t
25 weeks
of
OF
SELECTED
humidity
loss
storage
of
AND
and t e m p e r a t u r e
Detroit
(range
FRUITS
on
Red b e e t s
includes
VEGETABLES
after
results
for
3
storage
seasons). RH, %
Temperature °C
L o s s e s due t o decay, % of i n i t i a l weight
Rate of weight l o s s , %/30 d a y s
85-95
0-1
40-100
1 .,5 - 4 . ,0
98-100
0-1
25-35
0. . 1 - 0 . , 5
98-100
2-3
10-20
0. , 1 - 0 . .5
98-100
4-5
0-10
0, . 1 - 0 . , 5
Tab!e II.
Effect loss
relative
and on w e i g h t
celery o r more Vegetable cultivars
of
and
humidity loss
of
and C h i n e s e c a b b a g e
on d e c a y Brussels stored
at
and
trimming
sprouts, 0-l°C
cauliflower,
for
two
seasons. RH, %
Storage time, weeks
L o s s e s due t o d e c a y and t r i m m i n g , % of initial weight
Rate of weight l o s s , %/30 days
Brussels sprouts (Jade C r o s s )
9 0 - 95 9 8 - 100
11-12 11-12
4 0 - 100 2 0 - 30
3 . 4 - 5 . .2 0. 3 - 0 . ,4
C a u l i f1ower ( I m p e r i a l 1006, S u p e r J u n i o r and Self-blanche)
9 0 - 95 9 8 - 100
5-6 5-6
1 5 - 30 2 0 - 30
3 . 0 - 3 . .9 0. . 6 - 1 .,4
Celery ( U t a h 15 and Utah 5 2 - 7 0 )
9 0 - 95 9 8 - 100
11-12 11-12
7 0 - 75 4 5 - 65
2. . 2 - 3 . . 1 0. , 4 - 0 . . 5
Chinese cabbage (Michihli)
9 0 - 95 9 8 - 100
11-14 11-14
2 5 - 75 2 0 - 40
0. , 6 - 2 . .3 0- •0.2
8.
V A N D E N BERG
Maintaining Vegetable Quality During Storage
T a b ! e 111. E f f e c t
of atmospheric
on d e c a y cabbage
and t r i m m i n g after
(averages type Temperature, °C
c o m p o s i t i o n and t e m p e r a t u r e loss
and on w e i g h t
12 t o 30 weeks
of storage
f o r up t o 5 s t o r a g e
seasons,
loss
of green
a t 9 8 - 1 0 0 % RH "Evergreen"
cultivars).
Atmospheric L o s s e s due t o c o m p o s i t i o n , % decay and t r i m m i n g , 0 C0 % of i n i t i a l weight 9
9
0-1
>20
0-1
10
0-1 0-1
Rate o f weight loss, %/30 d a y s
20-30
(30
weeks)
0., 3 - 0 : ,7
11
40-70
(30
weeks)
0., 2 - 0 . ,5
6
5
30-50
(30
weeks)
0., 2 - 0 . ,5
1-3
5
10-30
(30
weeks)
0., 2 - 0 . , 5
20
20
